If you haven't already, I suggest that you read our earlier blog post on lumens, lux, and candella HERE to get up to speed with the terminology.
It is a fairly commonly held perception that 3 to 5 lux is amount of light that the average person needs on an object in order to quickly and reliably spot it on either a trail or a roadway. This is a figure that is independent of distance. This covers everything from tree branches, to the trail, to road markings, other pedestrians, or parked cars even, simply just how much light is needed on a target.
So with that number in mind we can start to think about situations where we need that much light in order to light up something enough to recognize it quickly. Lets say you are riding with a 1000 lumen light (actual lumens!) and there was a big rock on the trail 50 meters in front of you, this means we need a light that can light up that rock with 3 lux of illumination at 50 meters. We can use some math to figure out how much candella, or light intensity is needed from our light:
Iv(cd) = Ev(lx) × (d(m))2
Iv(cd) = 3(lx) × (50(m))2
Iv(cd) = 7,500 cd
Now, we can think of candella and lumens as linearly related, i.e. lumens is like a volume knob for candella. Twice as many lumens will increase the candella of that point twice as much. Easy right? So now lets say you were riding with a 2000 lumen light, so your candella is now 15,000 cd, logic would dictate that you should be able to see that rock from 100 meters away now right? Well let's look at the numbers:
Iv(cd) = Ev(lx) × (d(m))2
15,000(cd) = 3(lx) × (d(m))2
15,000(cd) / 3(lx) = (d(m))2
√(15,000(cd) / 3(lx)) = d(m)
d = 70.7 m
So instead of being able to see that rock from 100 meters out, you only gained a 41% improvement in actual spotting distance despite a 200% increase in the number of lumens in your light. This is part of why big lumen numbers are not key. What is more important is the quality of the beam pattern and how those candella intensities are spread out within your field of vision. These are not easy to report numbers, and have to rely more on wall shots and user reviews in order to figure out what is a quality beam pattern. If you see an ad for a light that doesn't show exactly what kind of beam pattern you are getting, run away, proper companies that do the work for developing a proper light will be happy to show you wall shots showing off how smooth and well illuminated their patterns are. For example, here is the Trail Edition beam pattern, can see how radically different it is from the typical bike light found.
If you have any other questions, always feel free to start up a chat!